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The Ars guide to advanced cooling: heatsink edition

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Heatsinks are the most basic form of cooling next to simple surface convection in today's PCs.? If you look inside the average machine, you'll probably find two or three heatsinks: on CPUs, video cards, and even the chipset of a motherboard.? Typically they appear as nothing fancy: hunks of aluminum with a large number of protrusions.? While there are different ways of manufacturing heatsinks, and different philosophies in the way they are shaped, the idea of all of them is the same: increase surface area to increase heat dissipation.

Not all heat sinks are created equal. ? Quite the opposite in fact.? In the first place, you need different heatsinks for different CPU interfaces.? A heatsink designed for a Slot 1 CPU will be completely useless for cooling a Socket 370 CPU.? But, more than that, heatsinks differ in their performance.? There is an entire subculture devoted to figuring out which heatsink will give the best results on with a given CPU.? For example, a lot of people like Alpha heatsinks, which are easy enough to get, but others swear by obscure heatsinks that were only ever produced for OEMs and are only ever available in lots of a dozen when some random retailer manages to get ahold of them.? It can get confusing at times, especially for a beginner, and knowing fact from fiction can also be a challenge for the experienced coolmeister. Hence, on the heels of our advanced cooling article on Peltiers, we decided to visit the other end of the cooling spectrum, and get down to some basics.

An Alpha heatsink, with the famous hexagonal pin

Heatsink theory

Meet Heat. AKA the enemy. Without delving into the fine distinctions made by physics between molecular movement, energy transference, and whatnot, let's just focus on what's clear: heat is "generated" by CPUs and other electronics, and when accumulated, it can cause damage. We want to get it away from our electronics as quickly as possible. Traditionally speaking, heat can be dissipated in a variety of ways, of which only two are relevant to this discussion. Interestingly, these two methods literally "encompass" the topic of heatsinks.

Now, the most important factor in a heatsink is, naturally, its ability to dissipate the largest amount of heat in the shortest amount of time.? This ability depends on a number of factors.? First is material. The vast majority of heatsinks are made out of aluminum.? Aluminum is an excellent conductor of heat, and relatively cheap. 'Conduction' is the primary method by which heat will be removed from your CPU, or other cooled device. Roughly speaking, conduction can be understood as the transfer of molecular kinetic energy between solids. If you stick your finger on a hot boilerplate, the extremely exited molecules in the boilerplate will collide with the molecules in your finger, gettin' them all excited as well, and hence displacing a bit of the energy. When choosing a conductor, we ideally want to go for something that can "absorb" a lot of heat, quickly.? This is why heatsinks are made of metal, and not cotton ;)

Those of you who have been following the industry know that aluminum has also been the primary material for interconnects on chips because of its conducting prowess.? You may also have noted that there is a movement towards copper interconnects replacing the aluminum ones. ? Copper is indeed a better conductor than aluminum.? So, you may ask, why aren't more heatsinks made out of copper?? For the same reason that processors have been using aluminum for the past 30 years: it's cheaper.? The amount of metal needed for CPU interconnects is minuscule compared to the amount one would need to make a pure copper heatsink.? Thus, aluminum.? Some manufacturers have tried to get the best of both worlds by combining the two: the plate that contacts the CPU is made out of copper, and the rest of the heatsink is made out of aluminum.

The second factor in heatsink effectiveness is, as mentioned above, surface area.? The protrusions function to make the exposed surface area many times greater than if the same amount of material was in a solid block.?Why? The greater the surface area exposed to the air, the greater the dissipation of heat for a given quantity of metal.?Here we meet our second friend in the heat dissipation process: convection. Convection is, for the purposes of this article, quite similar to conduction except that instead of rather efficient transfers of kinetic energy from dense solid to dense solid, we're now concerned with transferring energy into the air (or for some hardcore types, water). What makes convection different is the density of the substance in contact with the heatsink (air is far less dense, and slower to absorb heat if stagnant), and the fact that most solutions now use fans to force larger quantities of air over the surface of a heatsink.

Hence the importance of increasing surface area: whereas great amounts of heat can be transferred in 1.5 cubic centimeters of contact space on the bottom of your heatsink, that energy will require many more times the surface area to be effectively sent out into the air. The most common form in which the protrusions can be found is a series of rows and columns of thin rectangular protrusions.? Others are a thin piece of metal that has been folded back and forth across the surface of the heatsink.? Still others are a series of cylindrical protrusions.? And those are only the most common types.

???? A stock Intel heatsink with ???? a standard "fin" configuration.

The final factor is design.? This amorphous category covers both the particular arrangement of the extrusions and the quality of manufacture.? It is not enough that heat be able to radiate from the protrusions of the heatsink.? It also needs to get from the processor to those protrusions.? And then the protrusions, though they may involve a lot of surface area, won't be very much good if air is unable to easily travel across them.? Likewise, a heatsink doesn't do much good if it doesn't contact the CPU effectively.

By the way, if you're wondering about radiation, the quick story is that?it's not relevant to this discussion.? The heat dissipation differences between different color heatsinks are so similar once they're closed up inside your machine that they can be safely ignored.? Go ahead.? Use that blue heat sink.? We won't stop you.

So, given all that, how does one choose a heatsink?? Well, a lot of companies make a lot of claims.? Unfortunately, the only way to truly test these claims is to actually try the heatsinks out.? Luckily for the prospective cooling freak, there's lots of people in the same boat, some of whom have already done that testing and are more than happy to report on results.? In addition to reviews of various cooling technologies on sites like Ars, the overclocking/cooling community members will make their own judgments on various technologies and report back for all the world to see.

But first let us talk about those things that come together to ensure that any given heatsink is worth its weight in aluminum.